1. Field of the Invention
The present invention relates to a planar lightwave circuit comprising an optical transceiver integrated with a light-emitting element or a light-receiving element.
2. Description of the Related Art
The development of optical components is becoming increasingly important with advances in the optical communication technology. Above all, an optical transceiver has been contemplated to increase transmission speed and response speed, thereby increasing its communication capacity. A commonly used transceiver comprises a light-emitting element or a light-receiving element formed by using an optical semiconductor and an optical fiber for input or output that are optically coupled through a lens. In an optical receiver, for example, light emitted from an optical fiber at the input side focuses on the light-receiving element through the lens and is detected by direct detection (intensity detection).
For a modulation/demodulation processing technique in an optical transmission system, signal transmission using a phase modulation scheme has been widely practiced. A phase shift keying (PSK) scheme is a scheme for transmitting signals by modulating the optical phase. With this scheme, transmission capacity has been increasing exponentially by multilevel modulation.
In order to receive such PSK signals, detection of optical phase is required. A light-receiving element is capable of detecting intensity of signal light but is incapable of detecting optical phase, and thus a method for converting optical phase to optical intensity is required. Such a method can be a method for detecting phase difference by using optical interference. With this method, the signal light is interfered with another light (reference light) and the optical intensity of the interfering light is detected by a light-receiving element to obtain optical phase information. Here, the detection may be coherent detection using separately provided light source as reference light or differential detection for splitting a portion of the signal light itself as reference light to be interfered with the signal light. As described above, a recent PSK optical receiver requires an optical interferometer for converting phase information to intensity information by optical interference which is different from the conventional optical receiver using only an intensity modulation scheme.
Such an optical interferometer can be implemented by using a planar lightwave circuit (PLC). The planar lightwave circuit has an advantageous feature in terms of mass productivity, low cost, and high reliability, which allows to implement various optical interferometers. An optical delay line interferometer, a 90-degree hybrid circuit, etc. are implemented as the optical interferometer used in the PSK optical receiver for practical use. Such a planar lightwave circuit can be formed by glass deposition techniques such as a standard photolithography method, an etching technique, and flame hydrolysis deposition (FHD).
In view of a specific forming process, an underclad layer comprised mainly of silica glass and a core layer having a refractive index higher than that of a clad layer are firstly deposited on a substrate such as Si. Then, various waveguide patterns are formed on the core layer to finally embed the waveguide formed of the core layer in an overcladding layer. Such a process is performed to form a waveguide-type optical functional circuit. The signal light is confined in the waveguide formed via the above process and is propagated inside the planar lightwave circuit.
FIG. 1 illustrates a method for optically connecting a conventional planar lightwave circuit and an optical receiver. In view of the method for optically connecting a planar lightwave circuit in a PSK optical receiver and an optical receiver, the basic connection therebetween is a simple fiber connection as illustrated in FIG. 1. Here, a planar lightwave circuit 1 with each of its end connected to optical fibers 3a and 3b is interconnected with an optical receiver 2 having an input optical fiber 3b by optical fibers to provide optical coupling therebetween. The number of optical fibers used for optical coupling can be determined by the number of output lights outputted from the planar lightwave circuit which may be more than one. However, there has been a problem that the optical receiver using such optical fiber connection is increased in size. To get around this problem, the output of the planar lightwave circuit and the input of the optical receiver are optically coupled directly by using a lens to be integrated into one package for downsizing. The above-mentioned optical receiver in which a planar lightwave circuit and an optical receiver are optically coupled directly is called an integrated optical receiver.
A method for fixing the planar lightwave circuit becomes critical to implement the integrated optical receiver. In the case where the light outputted from the planar lightwave circuit is propagated in space to have optically coupled to the light-receiving element by a lens or the like, all the lights may not be received by the light-receiving element if there are changes in the positional relationships among the light emitting end, the lens, and the light-receiving element. Thus, this leads a problem of loss of light. In particular, the positions of the above are varied due to thermal expansions if there are changes in the temperature of the package for housing the optical receiver, ambient temperature, temperature of each element, etc. As a result, the above problem becomes more pronounced. Therefore, in order to carry out optical coupling with low loss, each positional relationship should not be varied at least in relative terms even if there is a change in the ambient temperature.
In particular, change in the shape of the planar lightwave circuit due to thermal expansion caused by change in the ambient temperature is substantially greater that of the light-receiving element. Further, the area of the planar lightwave circuit occupied in the optical receiver is larger than that of the light-receiving element by one to two orders of magnitude, and the shape change in the planar lightwave circuit due to thermal expansion is also greater than that in the light-receiving element by one to two orders of magnitude. In addition, there is a great difference in the thermal expansion coefficients between the substrate forming the planar lightwave circuit and the deposited thin glass, thereby causing significant warping due to thermal changes. Accordingly, the displacement of light emitted from the planar lightwave circuit and change in the emission angle with respect to the light-receiving element are even more problematic. These two changes bring about changes in the positions and angles of light emitted from the planar lightwave circuit, thereby causing displacement of an optical axis. The displacement of the optical axis causes degradation in optical coupling to the light-receiving element and losses in the optical coupling. For the implementation of the integrated optical receiver, it is critical that such displacement of the optical axis be resolved and be free from adverse effect.
FIG. 2 illustrates the inner structure of the conventional integrated optical receiver. A method for firmly fixing almost the entire bottom surface of the planar lightwave circuit is known to prevent the cause of aforementioned displacement of an optical axis due to the thermal changes. In the integrated optical receiver illustrated in FIG. 2, a planar lightwave circuit 13 forming an optical interferometer thereon as an optical functional circuit, a lens 14, and a light-receiving element 15 are respectively fixed to a base substrate 11 by fixing mounts 12a, 12b, and 12c serving as supporting members. An optical fiber 16 and the planar lightwave circuit 13 are connected through an optical fiber fixing component 17. In the integrated optical receiver, the light inputted from the optical fiber 16 is interfered in the planar lightwave circuit 13 to be coupled to the light-receiving element 15 by the lens 14.
The fixing mount 12a and the planar lightwave circuit 13 are fixed by an adhesive 18 or solder. Almost the entire bottom surface of the planar lightwave circuit 13 is firmly fixed to the fixing mount to limit the thermal expansion and warping changes. Further, the lens 14 and the light-receiving element 15 are also fixed to the fixing mounts to prevent displacement of an optical axis due to thermal changes.
The structure of FIG. 2 allows to substantially inhibit the displacement of an optical axis due to thermal changes while property change in the planar lightwave circuit due to thermal changes becomes prominent. As mentioned previously, the planar lightwave circuit 13 is composed of an Si substrate 13a and a silica glass layer 13b having a great difference in the thermal expansion coefficients therebetween. As a result, warping change and thermal expansion due to thermal changes become significant. In the structure illustrated in FIG. 2, the entire bottom surface of the planar lightwave circuit 13 is fixed to limit thermal expansion and warping changes.
Meanwhile, in such a structure, high thermal stress is generated between the Si substrate 13a and the silica glass layer 13b. The stress causes refractive index change inside the silica glass layer 13b through the photo-elastic effect. The optical interferometer formed in the planar lightwave circuit 13 has the length of the waveguide and the refractive index precisely adjusted to control the interference property. The refractive index change caused by the stress brings about changes in the equivalent circuit length and the property of the interferometer, thereby causing degradation in the property of the optical interferometer.
In this regard, the use of an elastic adhesive, a soft adhesive such as paste, or fixing paste as an adhesive 18 for limiting changes in the optical property by reducing thermal stresses (see, for example, Japanese Patent Application Laid-open No. 2009-175364) causes prominent effect of the aforementioned displacement of an optical axis and causes losses.